There are many known methods for generating chlorine dioxide, including both chemical and electrolytic processes. In the chemical processes, chlorine dioxide is typically generated from oxidizing chlorite or reducing chlorate. These chemistries often result in very corrosive and sometimes unsafe disinfecting by-products. Electrolytic processes typically generate chlorine gas to oxidize chlorite or generate a proton to reduce chlorates to chlorine dioxide. Although they achieve the desired end result of producing chlorine dioxide, these electrolytic cells can be very large in size and expensive. They may also contain fragile ion separation membranes, and may be too cumbersome for generating chlorine dioxide at the point of use. These prior art technologies are corrosive, contaminated with impurities, costly, or too slow in reaction rates for producing point-of-care products incorporated chlorine dioxide.
U.S. Pat. No. 2,323,593 discloses a process for the production of chlorine dioxide wherein chlorite ions are oxidized with persulfate ions in an aqueous reaction mixture. The reaction is taught as proceeding acceptably at ambient temperatures and at a reaction mixture pH of from 3 to 11. The patent teaches reacting the persulfate and chlorite in a molar ratio of persulfate to chlorite of from about 0.6:1 to about 0.75:1, and teaches that ratios of 1:2 or smaller should be avoided because they decrease the reaction rate. The chlorine gas is collected in an absorption tower by passing an inert gas through the reaction mixture. This is a costly and hazardous practice.
U.S. Pat. No. 6,171,485 discloses a process for the production of chlorine dioxide wherein chlorite ions are oxidized with an oxidizing agent, with the oxidizing agent being present in an amount between once and twice the stoichiometric amount necessary for oxidation of the chlorite to chlorine dioxide, and with the reaction solution at a pH of from 5.5 to 9.5, achieved through the presence of proton donors. Two solutions are employed, an aqueous chlorite solution and an aqueous oxidizing agent solution, mixed to form the reaction mixture that produces chlorine dioxide. A buffer substance is also preferably employed to form a buffering system active between pH 5.5 and 9.5 in the reaction mixture. This process was found to produce chlorine dioxide fairly rapidly, with a 95% conversion of chlorite to chlorite ions to chlorine dioxide occurring in about 12 days. With the inclusion of catalytic amounts of ions of transition metals present in the reaction mixture, even faster conversions are realized—on the order of about 98% conversion at 7 days. This patent focuses on producing chlorine dioxide for the treatment of water, and does not address the production of chlorine dioxide for surface treatment such as hard surfaces and biological surfaces.
Although the prior art teaches that chlorine dioxide may be generated from metallic chlorites reacted with oxidizers, proton donors, or combinations of these reaction initiators, these reactions produce relatively high concentrations of chlorine dioxide, greater than 1000 parts per million (ppm). This highly concentrated product is then immediately diluted for potable water purification, using acid buffers to control the reaction rate, or semi-permeable devices to control the rate of release and purity of chlorine dioxide delevered.
The present invention goes beyond the teaching of the prior art to provide a method for point-of-use generation of chlorine dioxide, wherein the chlorine dioxide is generated quickly and at acceptable concentrations to avoid subsequent dilutions. Additionally, the chlorine dioxide product is not corrosive or degrading to the surface to be treated.